This invention relates to turbofan engines and particularly to the exhaust nozzle control and to the core engine fuel control for optimizing thrust during steady state or transient conditions especially when the area of the exhaust nozzle is being selected for a given flight mode, as for example thrust augmentation by a thrust augmentor.
As is well known in the art, it is abundantly important for aircraft efficiency, specific fuel consumption and the like to achieve maximum thrust for a given flight mode. Since thrust cannot be measured directly, many attempts have been made to simulate or calculate thrust by measuring certain core engine operating parameters. Not only do such systems require instrumentation located in critical core engine locations, which may be undesirable, the smaller the tolerance band approximating the actual thrust being developed the more accurate the control will be, and the heretofore systems do not lend themselves to hold a small tolerance band.
I have found that I can achieve the highest thrust possible while being consistent with turbine temperature limits, fan flow stability limits and aircraft air inlet limits by closing the loop on different engine parameters in different regimes of the engines operational environment. This control operates in three preferred modes, namely, subsonic, transonic and supersonic. In the subsonic aircraft flight regime the loop is closed on fan pressure ratio by adjusting or trimming engine exhaust nozzle area. The loop is closed on fan rotor speed by adjusting or trimming core engine fuel flow. In the transonic aircraft flight regime the loop is closed on fan pressure ratio by adjusting exhaust nozzle area. The loop is closed on maximum turbine inlet temperature consistent with engine durability limits by adjusting core engine fuel flow. In the supersonic aircraft flight regime the loop is closed on engine airflow by adjusting engine exhaust nozzle area. The loop is closed on turbine inlet temperature by adjusting core engine fuel flow. It is to be understood that the terms subsonic, transonic and supersonic flight regimes only approximate the control's transitions between modes. The determination of the particular control mode is embodied in the invention and is a function of both aircraft and engine operating conditions. The transitions between the controls preferred modes occur without step changes in engine conditions. In both the exhaust nozzle area and core engine fuel flow control loops the smooth transitions are provided by selecting the maximum or minimum (as the case may be) error signals provided by the control logic. The transitions between the modes recognizes the interactions of engine and aircraft characteristics which combine to provide the resultant thrust; optimum thrust resulting in optimum aircraft performance.